The limit switch in general comprises a switch case housing, a basic switch mechanism having a push-in rod actuator, a head mounted on the switch case and housing, among others, a cam means for transforming a rotary motion into a linear motion, and an operating lever privotally connected to the head. Here, as the operating lever is angularly displaced by an object to be detected, such as a work, to an operating position against the biasing force of a return spring means, the angular displacement of the operating lever is transformed into a linear motion to drive a plunger supported by the switch case or head and thereby push in the rod actuator of the basic switch.
FIG. 7 is a partially exploded side elevation view showing the main part of a conventional limit switch.
As illustrated, a head 3 secured to the top of a switch case housing a basic switch mechanism has a rotary shaft 4 supported by journal bearing means 5,6 and an operating lever 7 is secured to an outer end 4a of the rotary shaft 4. Secured to the rotary shaft 4 are a first cam 12 and a second cam 13 in juxtaposition and a cam projection 12a of the first cam 12 is set for rotation, for example in the direction of arrowmark a in FIG. 8, while a cam projection 13a of the second cam 13 is set for rotation, for example in the direction of arrowmark b in FIG. 8. Installed through a spring holder 11 on the flat peripheral surface of the rotary shaft 4 is a spring means 14 for applying a returning force to the operating lever 7 via the rotary shaft 4.
At the bottom of the head 3, a plunger holder 15 is rotatably supported by a cover plate 10, with an annular retaining spring 8 having tongue members 8a, shown in FIG. 9, being interposed between the plunger holder 15 and the cover plate 10. A plunger 16 associated with the first and second cams 12,13 is pivotally supported by the plunger holder 15. A pair of steel balls 17,18 are interposed between the plunger 16 and the first and second cams 12,13 in such a manner that the balls are free to roll. In addition, the steel balls 17,18 are concentrically accommodated in a pair of holding orifices 21a, 21b, respectively, of a steel ball holder 21 secured to the bottom wall of the head 3, while a top wall 16g of the plunger 16 is formed with a cutout 16f for accepting either one of the steel balls 17,18 (FIG. 10).
Now, as the rotary shaft 4 is rotated in the direction of arrowmark a in FIG. 8 in response to rotation of the operating lever 7, the cams 12,13 overcome the biasing force of the spring means 14 to turn until the cam projection 12a is abutted against the steel ball 17, thus pushing the plunger 16 axially to turn the basic switch, not shown, ON, for instance.
On the other hand, when the rotary shaft 4 is rotated in the direction of arrowmark b in response to rotation of the operating lever 7, the other cam projection 13a is not abutted against the steel ball 18 because this steel ball 18 has been fitted into the cutout 16f, with the result that the plunger 16 is not axially driven and, hence, the basic switch is retained in OFF position.
Thus, the plunger 16 is axially driven to turn the basic switch ON only when the operating lever 7 causes the rotary shaft 4 to turn in the direction of arrow-mark a. The relative position of cam projections 12a,13a of the cams 12,13, the steel balls 17,18 and the cutout 16f of the plunger 16 in the above situation is diagrammatically illustrated in FIG. 11 (A).
When the plunger 16 is swung about its axis in the direction of arrowmark c or d into the position illustrated in FIG. 11 (B), the reverse of the above situation holds. Thus, the plunger 16 is driven axially to turn the basic switch ON only when the operating lever 7 is rotated in the direction of arrowmark b.
Furthermore, when the plunger 16 is set in the position indicated in FIG. 11 (C), the following relation holds. Thus, the plunger 16 can be axially driven to turn the switch ON by rotating the operating lever 7 in whichever of the directions shown by arrow-marks a and b.
In the above arrangement, a variety of axial drives of the plunger 16 can be achieved by changing relative position of the cutout 16f of plunger 16 with respect to the cams 12,13 but in order to achieve such results, the steel balls 17,18 and steel ball holder 21 are essential and this means not only a large number of parts required but also a complication of assembling work.